AREA of the trapezoid === 2 m x (1.4 + .6)/2 = 2 m^2
VOLUME of the pond = Area of trapezoid by width = 2 m^2 x 1 m = 2 m^3
In 30 minutes volume lost = .2m x 2 m x 1 m / 30 min = ..4 m^3 /30 min
2 m^3 / .4 m^3/ 30 min = 150 minutes to completely drain
Question is unclear : it will take 150 minutes to completely drain from the start or 120 from the time 20 cm has drained .